We gots #PCl_3#, and around the central phosphorus atom there are #3xx"bonding electron pairs"#, the #P-Cl# bonds, and a #"phosphorus-centred"# lone pair.
To a first approximation, these electron pairs arrange themselves in a tetrahedron........with ideal bond angles of #109.5^@#. But we describe molecular geometry on the basis of the geometry of the ATOMS, not on the electron pairs. The symmetry descends from tetrahedral to trigonal pyramidal with #/_Cl-P-Cl# angles of approx. #100-103^@# due to the disproportionate influence of the phosphorus lone pair.
